Step ordering during fractional-layer superlattice growth on GaAs(001) vicinal surfaces by metalorganic chemical vapor deposition

Step ordering during fractional-layer superlattice (FLS) growth is observed by trans- mission electron microscopy. A cross-sectional transmission electron micrograph of an (AlAs)1/2(GaAs)1/2FLS shows that the steps of unequal spacings observed on a GaAs surface develop a sequence of uniformly spaced steps of single monolayer height during the growth. Comparison of the observed FLS structure with a numerical simulation based on the assumption of unequal incorporation probabilities of adatoms into up steps and down steps shows that a net flux into the up steps 2%–4% larger than that into the down steps is enough to reproduce the FLS growth. It is demonstrated that the FLS growth provides a unique opportunity to observe the step ordering process directly and determine the anisotropy of diffusion quantitatively.